An ignition coil used in an ignition device of an internal-combustion engine supplies a direct current to a primary coil and excites a high voltage in a secondary coil by conducting and blocking the electric current. In other words, the magnetic flux generated by having the electric current flow through the primary coil is guided to the secondary coil using an iron core and the magnetic flux is changed to generate a high voltage.
In order to have the secondary coil generate a high voltage in an efficient manner, and in order to reduce the size of the ignition coil and put direct ignition into practical use, using a closed magnetic ignition coil is becoming the mainstream in internal-combustion engines.
The closed magnetic ignition coil includes an iron core that constitutes a magnetic circuit through which a magnetic flux generated by a primary coil permeates.
The iron core penetrates a center hole of the primary coil, is extended to an outer peripheral side of the primary coil, is formed in an annular shape so as to connect both winding ends of the primary coil, returns the magnetic flux emitted from the primary coil to the primary coil once more, suppresses the attenuation of the magnetic flux and interlinks with the secondary coil, so that a high voltage is induced efficiently (see PTL 1, for example).
FIG. 4 is an explanatory drawing illustrating a magnetic circuit formed in a conventional ignition coil for an internal-combustion engine. The drawing illustrates a schematic longitudinal section of a conventional ignition coil 100, and the illustration of a secondary coil and the like is omitted in order to clearly illustrate the magnetic circuit and a primary coil.
The ignition coil 100 includes a center core 102 that is inserted into a center hole of a primary coil 101, a side core 103 that is formed so as to surround both lateral sides of the center core 102, and a permanent magnet 104 disposed between a one side portion 103a of the side core 103 and the center core 102.
Note that the magnetic circuit described above is formed by the center core 102 and the side core 103.
In the drawing, the center core 102 directly connects an end portion 102a on a lower side to the side core 103.
The end portion 102b on an upper side of the center core 102 is in contact with a permanent magnet 104 that supplies a bias magnetic field, and forms a magnetic circuit that is connected to the one side portion 103a of the side core 103 with the permanent magnet 104 interposed therebetween.
The end portion 102b of the center core 102 is formed large so as to obtain a sufficient area in contact with the permanent magnet 104, and the center core 102 is formed in a T-shape. The T-shaped vertical portion is inserted into the center hole of the primary coil 101, and the T-shaped horizontal portion is, as described above, in contact with the permanent magnet 104.
The solid line arrows illustrated in FIG. 4 depict a magnetic flux C generated when a primary current, which is a direct current, flows through the primary coil 101, and the broken line arrows depict the magnetic flux D emitted from the permanent magnet 104.
When the primary current flows through the primary coil 101, the magnetic flux C generated by the primary coil 101 permeates inside the magnetic circuit in the direction indicated by the solid line arrows.
The magnetic flux D depicts the bias magnetic field described above and permeates inside the magnetic circuit in a direction opposite to that of the magnetic flux C.
The magnetic flux C permeating the center core 102 permeates the permanent magnet 104 and reaches the side core 103 (the one side portion 103a). Accordingly, a magnetic reluctance caused by the permanent magnet 104 acts on the magnetic flux C.